During surgical procedures it is sometimes necessary to view internal body parts through a small opening which may be on the order of about 10 millimeters in diameter. In particular, it is now common to perform surgical procedures without making a large incision by operating through a plurality of rigid tubes inserted into the body. Procedures of this type, known as endoscopic surgical procedures, provide distinct advantages over traditional surgical procedures in terms of the length and associated cost of the patient's hospital stay and recuperation period. Equally as significant is the substantial reduction in patient discomfort. Instruments for inserting a rigid tube or cannula into the body are disclosed in U.S. Pat. Nos. 4,601,710; 4,654,030 and 4,902,280.
In order to inspect internal parts of the body and/or perform surgery in this fashion, it is necessary for the surgeon to be able to view inside the body without any need for a general opening. The generally accepted method for accomplishing this result is to provide an optical illumination and viewing apparatus known as an endoscope, which is insertable through a cannula. The surgeon may view the image directly through an eyepiece or, more commonly, by projecting the image onto a television screen.
High quality reusable endoscopes have long been known and provide the standard of excellence with respect to image quality. Indeed, the surgical community has come to expect image quality on the order of that provided by reusable endoscopes and will not accept inferior image quality. Unfortunately, however, reusable rigid endoscopes are expensive to purchase and require a high level of maintenance and care and experience degradation in optical image quality with time. Thus, reusable endoscopes must be handled carefully and must be cleaned thoroughly after each use. Cleaning the endoscope requires special training and adds to the cost of the surgical procedure by consuming hospital personnel time.
In order to reduce cost and the potential for transmission of disease or infection there presently is a trend toward use of disposable instruments. Many types of disposable surgical instruments have been provided, but an economical disposable rigid endoscope which provides acceptable image quality has not heretofore been available.
Yamashita U.S. Pat. No. 4,036,218 discloses an endoscope assembled without the use of spacer tubes.
Hoogland U.S. Pat. Nos. 4,545,652 and 4,575,195 disclose flat field lenses for an endoscope wherein Petzval sum correction is dominated by the use of index difference. Hoogland discloses a triplet lens assembly consisting of injection molded polystyrene end elements and a crown glass central element. The indices of refraction and powers of the elements are determined by requiring the Petzval sum to vanish and further requiring a given overall power. The dispersion of the materials are chosen to correct chromatic aberration.
Fantone U.S. Pat. No. 4,784,118 represents one attempt to provide a disposable endoscope, wherein the endoscope light pipe and the objective, relay and viewing lens assemblies all are fabricated of a polymeric material which lends itself to injection molding. Fantone discloses an objective lens system including a plano-concave negative lens and a double convex positive lens to provide a relatively short focal length with a field of view on the order of 60.degree. to 70.degree.. The objective lens system also includes a double convex field lens to reduce or eliminate vignetting at the edge of the field of view. According to Fantone, all concave or convex surfaces of the foregoing lenses preferably are aspheric, and all of the lenses are made, preferably by injection molding, of a polymeric material such as acrylic, polystyrene, polycarbonate or SAN, with acrylic preferred. A relay lens assembly consists of a plurality of injection molded polymeric rod lenses arranged end to end to transmit the image from the objective lens assembly to the proximal end of the endoscope. Fantone's preferred molded polymeric rod lenses are of double convex configuration having entrant and exit refracting surfaces of the same focal length, so that the image from the objective lens assembly is collimated and refocused several times during the relay. A viewing lens system includes a plano concave post rod lens and a strong positive lens, with either a window or a negative lens therebetween. The lens assembly is substantially uncorrected for axial color. Illumination of the body cavity is achieved by disposing a light pipe within a support tube so as to cradle the lens assembly. The light pipe may be fabricated from a molded polymeric material or glass or plastic fibers.
Leiner U.S. Pat. No. 4,964,710 points out several deficiencies in the structure disclosed by Fantone, and proposes a rigid endoscope comprised of a hybrid system incorporating glass plano cylinders with flat polished end faces disposed between molded plastic curved surface lenses having a thickness on the same order of magnitude as their diameter. Leiner further proposes that the plastic lenses can be made of two different types of plastic to allow for the correction of chromatic aberration. Leiner asserts that the plano glass cylinders can be economically made in large quantities, while the plastic lenses can be economically made by known injection molding processes. Like Fantone, Leiner discloses objective, relay and ocular lens assemblies. The objective lens system includes a field widening lens, a prism and objective lenses. These elements may be made of molded plastic, as taught by Fantone, or they may be made of conventional ground glass. The Leiner relay lens system comprises an even number of molded polymeric curved surface lenses. Leiner shows each plastic lens assembly as a lens pair and states that the lenses of each pair may be made of two different types of plastic material.
Notwithstanding the disclosures of Fantone and Leiner, there remains a need for an economical disposable endoscope which attains high quality imaging. Fantone and Leiner each make use of molded plastic optical components in an attempt to reduce cost, but the number of appropriate plastic materials is few, which limits the freedom of the optical designer to select from a variety of materials to reduce or eliminate chromatic aberration. Furthermore, the optical design disclosed by Fantone tends to focus any manufacturing defects or flaws at the image surface, and the paired lens configuration proposed by Leiner is not conducive to achieving reduced field curvature.
Therefore, a relatively low cost disposable endoscope is needed which provides the optical designer with greater degrees of freedom in selecting lens materials to correct for chromatic and other aberrations and which incorporates improved optical design so as to eliminate real or potential imaging problems of Fantone and/or Leiner.